The invention relates to a device for doubling the frequency of a light wave, in which device a fundamental light wave is guided through a nonlinear optical medium while forming a second harmonic wave, said nonlinear optical medium being formed from a polymer network and elements having a great hyperpolarizability.
The device according to the invention can be used, for example, in combination with a semiconductor laser light source generating red light, as a compact blue light source for optical storage of information and for telecommunication and laser printers. By virtue of the use of shortwave light, the information density can be increased and the possibilities of writing and erasing information are extended as a result of the larger photon energy.
In U.S. Pat. No. 4,886,339 a description is given of such a device for doubling the frequency of a light wave, said device comprising a nonlinear optical medium consisting of non-centrosymmetric molecular dipoles which are polarly oriented in a polymer network. The molecular dipoles exhibit a great hyperpolarizability. The use of a polymer network has the advantage that the chiral structure of the medium and the orientation of the hyperpolarizable elements can be fixed permanently, polymer networks being less sensitive to changes in temperature than the often used linear polymers having hyperpolarizable molecule groups in side chains.
In devices for doubling the frequency of a light wave the problem arises that the nonlinear optical medium has different refractive indices for the fundamental wave and the second harmonic wave, thus bringing about increasing phase differences between both waves when light is guided through the medium. Owing to destructive interference, a large part of the intensity of the generated blue light is lost. A possible solution to this problem is the use of materials having an anomalous dispersion or the use of birefringent material, the ordinary refractive index of which materials at one wavelength should be equal to the extraordinary refractive index at the other wavelength. Another solution which is known per se is the use of waveguides having spatially periodic structures. In the latter solution, the nonlinear optical properties in alternate regions are selected such that in regions where the fundamental light wave and the second harmonic wave are out of phase, the second harmonic wave is generated to a reduced degree or not at all, so that no complete extinction takes place. It is even possible to change the nonlinear optical properties in such regions in a manner such that the generated blue light is of the opposite phase (domain inversion), so that a further intensification of the blue light takes place.
In PCT Patent Application WO 90/04807 a description is given of such a (domain inversion) device in which domains of opposite ferroelectric polarity are manufactured under a mask layer in an inorganic nonlinear optical material by means of annealing. Due to its destructive character, said method is less suitable for use with polymer materials.